Perpendicular magnetic recording system is adopted as a technique for increasing the magnetic recording density. A perpendicular magnetic recording medium at least comprises a non-magnetic substrate, and a magnetic recording layer formed of a hard-magnetic material. Optionally, the perpendicular magnetic recording medium may further comprise: a soft-magnetic under layer playing a role in concentrating the magnetic flux generated by a magnetic head onto the magnetic recording layer; an interlayer for orienting the hard-magnetic material in the magnetic recording layer in an intended direction; a protective film for protecting the surface of the magnetic recording layer; and the like.
Hitherto, magnetic layers formed of CoCr-based unordered alloys such as CoCrPt have been investigated as a metallic magnetic material for the perpendicular magnetic recording medium. For the purpose of further increasing the recording density of the perpendicular magnetic recording medium, an urgent need for reduction in the grain diameter of the magnetic crystal grains in the magnetic layer arises in recent years. On the other hand, reduction in the grain diameter of the magnetic crystal grains leads to a decrease in thermal stability of the recorded magnetization (also referred to as recorded signals). Thus, the magnetic crystal grains in the magnetic layer need to be formed of materials with higher magnetocrystalline anisotropies, in order to compensate the decrease in thermal stability due to the reduction in the grain diameter of the magnetic crystal grains.
L10 type ordered alloys as the materials having the required higher magnetocrystalline anisotropies and production method thereof have been proposed. The L10 type ordered alloys include FePt, CoPt, FePd, CoPd, and the like. It is necessary to increase the degree of order by pre-heating or post-heating, when the L10 type ordered alloys are used. Japanese Patent Laid-Open No. 2012-48784 proposes a method for producing a perpendicular magnetic recording medium having a magnetic recording layer of a desired thickness, by repeating the steps of heating a substrate and depositing a film of the L10 type ordered alloy having a small thickness, in order to prevent reduction in the degree of order of the L10 type ordered alloy due to a drop in the substrate temperature (see PTL1).
On the other hand, reduction in the sizes of the magnetic crystal grains means reduction in the cross-sectional areas of the crystal magnetic grains having a certain height, since the thickness of the magnetic recording layer is basically uniform in in-plane directions of the magnetic recording medium. Therefore, a diamagnetic field acting on the magnetic crystal grains themselves decreases, whereas a magnetic field required for switching the magnetization of the magnetic crystal grains (magnetic switching field) increases. As described above, the improvement of the recording density implies that a larger magnetic field is required for writing a magnetization (or, recording signals), in view of the shape of the magnetic crystal grains. In other words, a problem of deterioration of the writing capability of the magnetic recording medium becomes apparent, as the recording density increases.
Chen et al. proposes a magnetic recording medium having a graded magnetic recording layer consisting of a bottom layer having a large magnetic anisotropy constant (Ku), a middle layer having an intermediate Ku, and a top layer having a low Ku in this order, as a trial to improve writing capability of the magnetic recording medium while maintaining the thermal stability of the recorded signals (see NPL1). Here, the bottom layer, the middle layer and the top layer have a granular structure consisting of L10 type FePt ordered alloy and a C grain boundary. Further, the Ku's of the respective layers are controlled by adjusting the degree of order of the L10 type FePt ordered alloy by changing the temperatures when forming the bottom, middle and top layers. Chen et al. reports that the coercive force of the above-described three layer construction decreases to 5.9 kOe (about 470 A/mm), whereas the sole high Ku layer has a coercive force of 11.4 kOe (about 907 A/mm).
Besides, Zha et al. proposes a graded magnetic layer having a magnetic recording layer consisting of (FePt)100-xCux alloy wherein x, the content of Cu, monotonously decreases from 30 at the bottom to 0 at the top (see NPL2). Zha et al. reports that the graded magnetic layer consisting of (FePt)100-xCux alloy (x being monotonously decrease from 30 to 0) exhibits a coercive force of 5.67 kOe (about 451 A/mm), whereas a magnetic layer consisting of (Fe53Pt47)85Cu15 alloy and having uniform composition exhibits 7.21 kOe (about 574 A/mm).
Further, International Patent Publication No. WO 2012/105908 discloses ion implantation into a magnetic recording medium, and thereby forming a graded magnetic recording layer wherein the peak of amount of the implanted ions is at the top of the magnetic recording layer, and the amount of the implanted ions decreases in the depth direction in a graded manner. Here, the implanting ion is selected from the group consisting of He+, C+, N2+, Ar+, Co+, and Sb+.